Exhaust gas control device for internal combustion engine and control method thereof

ABSTRACT

An exhaust gas control device for an internal combustion engine includes an estimation unit configured to estimate a temperature of a catalyst on the basis of an acquired operation state of the internal combustion engine and a difference between a lean air-fuel ratio and a rich air-fuel ratio which are set as target air-fuel ratios during execution of catalyst regeneration control, a determination unit configured to determine whether the estimated temperature of the catalyst is higher than a threshold value during execution of the catalyst regeneration control, and a prohibition unit configured to prohibit the catalyst regeneration control when it is determined that the estimated temperature of the catalyst is higher than the threshold value during execution of the catalyst regeneration control.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2016-075168 filed on Apr. 4, 2016 including the specification, drawings and abstract is incorporated herein by reference in its entirety.

BACKGROUND 1. Technical Field

The disclosure relates to an exhaust gas control device for an internal combustion engine and a control method thereof.

2. Description of Related Art

A technique of increasing a temperature of a catalyst by so-called dither control of setting a target air-fuel ratio of one cylinder among a plurality of cylinders of an internal combustion engine to a rich air-fuel ratio and setting target air-fuel ratios of the other cylinders to a lean air-fuel ratio is known (for example, see Japanese Patent Application Publication No. 9-088663 (JP 9-088663 A)).

SUMMARY

Regenerating a catalyst by increasing the temperature of the catalyst up to a temperature range higher than a temperature range in which the catalyst is activated by such a technique can be considered.

Here, the rich air-fuel ratio and the lean air-fuel ratio which are set as the target air-fuel ratios in the above-mentioned technique are set to be variable depending on an operation state of the internal combustion engine to decrease an influence on drivability. Accordingly, while the control of regenerating a catalyst is performed, there is a possibility that the temperature of the catalyst will excessively increase over the temperature range required for regenerating the catalyst depending on the operation state of the internal combustion engine or the set rich air-fuel ratio and lean air-fuel ratio.

Therefore, the disclosure provides an exhaust gas control device for an internal combustion engine that suppresses an excessive increase in temperature of a catalyst during execution of catalyst regeneration control of setting a target air-fuel ratio of one cylinder to a rich air-fuel ratio and setting target air-fuel ratios of the other cylinders to a lean air-fuel ratio and a control method thereof.

The above-mentioned problem can be solved by an exhaust gas control device for an internal combustion engine, the exhaust gas control device including: a catalyst configured to purify exhaust gas discharged from a plurality of cylinders of the internal combustion engine; an acquisition unit configured to acquire an operation state of the internal combustion engine; a control unit configured to perform catalyst regeneration control, the catalyst regeneration control being control of increasing a temperature of the catalyst to regenerate the catalyst by setting a target air-fuel ratio of at least one cylinder among the plurality of cylinders to a rich air-fuel ratio which is lower than a stoichiometric air-fuel ratio and setting target air-fuel ratios of the other cylinders among the plurality of cylinders to a lean air-fuel ratio which is higher than the stoichiometric air-fuel ratio; an estimation unit configured to estimate the temperature of the catalyst on the basis of the acquired operation state of the internal combustion engine and a difference between the lean air-fuel ratio and the rich air-fuel ratio which are set as the target air-fuel ratios during execution of the catalyst regeneration control; a determination unit configured to determine whether the estimated temperature of the catalyst is higher than a threshold value during execution of the catalyst regeneration control; and a prohibition unit configured to prohibit the catalyst regeneration control when it is determined that the estimated temperature of the catalyst is higher than the threshold value during execution of the catalyst regeneration control. The aspect of the disclosure may be defined as follows. An exhaust gas control device for an internal combustion engine, the exhaust gas control device including: a catalyst configured to purify exhaust gas discharged from a plurality of cylinders of the internal combustion engine; and an electronic control unit configured to i) acquire an operation state of the internal combustion engine, ii) perform catalyst regeneration control, the catalyst regeneration control being control of increasing a temperature of the catalyst to regenerate the catalyst by setting a target air-fuel ratio of at least one cylinder among the plurality of cylinders to a rich air-fuel ratio which is lower than a stoichiometric air-fuel ratio and setting target air-fuel ratios of the other cylinders among the plurality of cylinders to a lean air-fuel ratio which is higher than the stoichiometric air-fuel ratio, iii) estimate the temperature of the catalyst based on the acquired operation state of the internal combustion engine and a difference between the lean air-fuel ratio and the rich air-fuel ratio which are set as the target air-fuel ratios during execution of the catalyst regeneration control, iv) determine whether or not the estimated temperature of the catalyst is higher than a threshold value during execution of the catalyst regeneration control, and v) prohibit the catalyst regeneration control when the electronic control unit determines that the estimated temperature of the catalyst is higher than the threshold value during execution of the catalyst regeneration control. A control method for an exhaust gas control device for an internal combustion engine, the exhaust gas control device including a catalyst configured to purify exhaust gas discharged from a plurality of cylinders of the internal combustion engine and an electronic control unit, the control method including: i) acquiring an operation state of the internal combustion engine by the electronic control unit; ii) performing catalyst regeneration control by the electronic control unit, the catalyst regeneration control being control of increasing a temperature of the catalyst to regenerate the catalyst by setting a target air-fuel ratio of at least one cylinder among the plurality of cylinders to a rich air-fuel ratio which is lower than a stoichiometric air-fuel ratio and setting target air-fuel ratios of the other cylinders among the plurality of cylinders to a lean air-fuel ratio which is higher than the stoichiometric air-fuel ratio; iii) estimating, by the electronic control unit, the temperature of the catalyst based on the acquired operation state of the internal combustion engine and a difference between the lean air-fuel ratio and the rich air-fuel ratio which are set as the target air-fuel ratios during execution of the catalyst regeneration control; iv) determining, by the electronic control unit, whether or not the estimated temperature of the catalyst is higher than a threshold value during execution of the catalyst regeneration control; and v) prohibiting, by the electronic control unit, the catalyst regeneration control when the electronic control unit determines that the estimated temperature of the catalyst is higher than the threshold value during execution of the catalyst regeneration control.

The temperature of the catalyst is accurately estimated on the basis of the operation state of the internal combustion engine and the magnitude of the difference between the lean air-fuel ratio and the rich air-fuel ratio which are set as the target air-fuel ratios during execution of the catalyst regeneration control. When the estimated temperature of the catalyst is higher than the threshold value, the catalyst regeneration control is prohibited and thus an excessive increase in temperature of the catalyst is suppressed.

The prohibition unit may be configured to prohibit the catalyst regeneration control in a predetermined period when it is determined that the estimated temperature of the catalyst is higher than the threshold value during execution of the catalyst regeneration control.

According to the disclosure, it is possible to provide an exhaust gas control device for an internal combustion engine that suppresses an excessive increase in temperature of a catalyst during execution of catalyst regeneration control of setting a target air-fuel ratio of one cylinder to a rich air-fuel ratio and setting target air-fuel ratios of the other cylinders to a lean air-fuel ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance of exemplary embodiments of the disclosure will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG. 1 is a diagram schematically illustrating a configuration of an exhaust gas control device;

FIG. 2 is a flowchart illustrating an example of regeneration prohibition control which is performed by an ECU;

FIGS. 3A and 3B are diagrams illustrating an example of a catalyst temperature map;

FIG. 4 is a timing chart illustrating an example of the regeneration prohibition control;

FIG. 5 is a flowchart illustrating a modified example of the regeneration prohibition control which is performed by the ECU; and

FIG. 6 is a timing chart illustrating a modified example of the regeneration prohibition control.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 is a diagram schematically illustrating a configuration of an exhaust gas control device 1 (an exhaust gas control device for an internal combustion engine). As illustrated in FIG. 1, the exhaust gas control device 1 includes a three-way catalyst 31 that purifies exhaust gas of an internal combustion engine 20. The internal combustion engine 20 combusts an air-fuel mixture in a combustion chamber 23 in a cylinder block 21 to cause a piston 24 to reciprocate. The internal combustion engine 20 is an in-line four cylinder gasoline engine, but is not limited thereto as long as it includes a plurality of cylinders and may be, for example, a diesel engine.

An intake valve Vi that opens and closes an intake port and an exhaust valve Ve that opens and closes an exhaust port are provided for each cylinder in a cylinder head of the internal combustion engine 20. An ignition plug 27 that ignites the air-fuel mixture in the combustion chamber 23 is attached to a top of the cylinder head for each cylinder.

The intake port of each cylinder is connected to a surge tank 18 via a branch pipe for each cylinder. An intake pipe 10 is connected to an upstream side of the surge tank 18 and an air cleaner 19 is disposed at an upstream end of the intake pipe 10. The intake pipe 10 is provided with an airflow meter 15 that detects an amount of intake air and an electronically controlled throttle valve 13 sequentially from the upstream side.

An injector 12 that injects fuel into the intake port is installed in the intake port of each cylinder. The fuel injected from the injector 12 is mixed with intake air to form an air-fuel mixture, and the air-fuel mixture is suctioned into the combustion chamber 23 when the intake valve Vi is opened, is compressed by the piston 24, and is ignited and combusted by the ignition plug 27.

On the other hand, the exhaust port of each cylinder is connected to an exhaust pipe 30 via a branch pipe for each cylinder. The three-way catalyst 31 is provided in the exhaust pipe 30. The three-way catalyst 31 has an oxygen occlusion capacity and purifies NOx, HC, and CO. In the three-way catalyst 31, one or more catalyst layers including a catalyst carrier such as alumina (Al₂O₃) and a catalyst metal such as platinum (Pt), palladium (Pd), or rhodium (Rh) carried by the catalyst carrier are formed on a substrate of cordierite or the like, particularly, a honeycomb substrate. The three-way catalyst 31 is an example of a catalyst that purifies exhaust gas discharged from a plurality of cylinders of the internal combustion engine 20 and may be an oxidation catalyst or a gasoline particulate filter coated with the oxidation catalyst.

An air-fuel ratio sensor 33 that detects an air-fuel ratio of exhaust gas is installed upstream from the three-way catalyst 31. The air-fuel ratio sensor 33 is a so-called wide-area air-fuel ratio sensor, which can continuously detect an air-fuel ratio over a relatively wide area and output a signal of a value proportional to the air-fuel ratio.

The exhaust gas control device 1 includes an electronic control unit (ECU) 50. The ECU 50 includes a central processing unit (CPU), a random access memory (RAM), a read only memory (ROM), and a storage unit. The ECU 50 performs various types of control by executing a program stored in the ROM or the storage unit. The ECU 50 performs regeneration prohibition control to be described later. The regeneration prohibition control is performed by an acquisition unit, a control unit, an estimation unit, a determination unit, and a prohibition unit of the ECU 50, which are functionally realized by the CPU, the ROM, and the RAM. Details thereof will be described later.

The ignition plug 27, the throttle valve 13, the injector 12, and the like are electrically connected to the ECU 50. In addition to the airflow meter 15, the air-fuel ratio sensor 33, and a crank angle sensor 25 that detects a crank angle of the internal combustion engine 20, an accelerator opening level sensor 11 that detects an accelerator opening level or various other sensors are electrically connected to the ECU 50 via an A/D converter or the like which is not illustrated. The ECU 50 controls the ignition plug 27, the throttle valve 13, the injector 12, and the like to acquire desired output power on the basis of detection values of various sensors, and controls an ignition timing, an amount of fuel injected, a fuel injection timing, a throttle opening level, and the like.

Setting of a target air-fuel ratio by the ECU 50 will be described below. In a normal state in which catalyst regeneration control to be described later is not performed, the target air-fuel ratio is set on the basis of a normal air-fuel ratio map based on an engine speed and an engine load of the internal combustion engine 20. The normal air-fuel ratio map is acquired by experiment in advance and is stored in the ROM of the ECU 50.

For example, the target air-fuel ratio is set to a stoichiometric air-fuel ratio in a low-speed and low-load area and is set to be richer than the stoichiometric air-fuel ratio in a high-speed and high-load area. When the target air-fuel ratio is set, an amount of fuel injected into each cylinder is feedback-controlled such that the air-fuel ratio detected by the air-fuel ratio sensor 33 reaches the target air-fuel ratio. The target air-fuel ratio based on the engine speed and the engine load may be calculated using a calculation equation instead of the normal air-fuel ratio map.

The ECU 50 performs catalyst regeneration control of removing a sulfur compound (SO_(x)) deposited on the three-way catalyst 31 to regenerate purification capability of the three-way catalyst 31 by increasing the temperature of the three-way catalyst 31 to within a predetermined temperature range. In the catalyst regeneration control, so-called dither control of setting a target air-fuel ratio of one cylinder among a plurality of cylinders to a rich air-fuel ratio which is lower than the stoichiometric air-fuel ratio and setting target air-fuel ratios in the other three cylinders to a lean air-fuel ratio which is higher than the stoichiometric air-fuel ratio is performed. An average of the target air-fuel ratios of all the cylinders is set to be the stoichiometric air-fuel ratio.

The target air-fuel ratios in the catalyst regeneration control are similarly set on the basis of a regeneration air-fuel ratio map based on the engine speed and the engine load. The regeneration air-fuel ratio map is acquired by experiment in advance and is stored in the ROM of the ECU 50. For example, the rich air-fuel ratio is set to range from 9 to 12 and the lean air-fuel ratio is set to range from 15 to 16. As the engine speed and the engine load become greater, the rich air-fuel ratio is set to be smaller and the lean air-fuel ratio is set to be larger.

The rich air-fuel ratio and the lean air-fuel ratio which are set as the target air-fuel ratios on the basis of the regeneration air-fuel ratio map are set to be variable depending on the engine speed and the engine load of the internal combustion engine 20 within a range in which an influence on drivability is small. The target air-fuel ratios in the catalyst regeneration control based on the engine speed and the engine load may be calculated using a calculation equation instead of the regeneration air-fuel ratio map. The catalyst regeneration control is not performed in an idle operation state or when the accelerator opening level is zero.

When the catalyst regeneration control is performed as described above, surplus fuel discharged from a cylinder of which the target air-fuel ratio is set to the rich air-fuel ratio is attached to the three-way catalyst 31 and is combusted in a lean atmosphere due to exhaust gas discharged at the lean air-fuel ratio. Accordingly, the temperature of the three-way catalyst 31 increases to remove SO_(x).

However, during execution of the catalyst regeneration control in which the three-way catalyst 31 is maintained at a high temperature, there is a possibility of the temperature of the three-way catalyst 31 excessively increasing over the temperature range necessary for regeneration depending on the operation state of the internal combustion engine 20 or the rich air-fuel ratio and the lean air-fuel ratio which are set as the target air-fuel ratios. Therefore, the ECU 50 performs regeneration prohibition control of prohibiting the catalyst regeneration control during execution of the catalyst regeneration control.

FIG. 2 is a flowchart illustrating an example of the regeneration prohibition control which is performed by the ECU 50. The control illustrated in FIG. 2 is repeatedly performed with a predetermined cycle. The ECU 50 determines whether the catalyst regeneration control is being performed (Step S1) and this control ends when the determination result is negative.

When the determination result in Step S1 is affirmative, the ECU 50 acquires the engine speed and the engine load of the internal combustion engine 20 (Step S3). Specifically, the engine speed is acquired on the basis of an output value from the crank angle sensor 25, and the engine load is acquired on the basis of an output value from the accelerator opening level sensor 11. The process of Step S3 is an example of the process which is performed by the acquisition unit that acquires the operation state of the internal combustion engine 20.

Then, the ECU 50 acquires a magnitude of a difference between the lean air-fuel ratio and the rich air-fuel ratio which are set as the target air-fuel ratios in the catalyst regeneration control as an air-fuel ratio difference (Step S5). Specifically, a value obtained by subtracting the rich air-fuel ratio from the lean air-fuel ratio is acquired as the air-fuel ratio difference. The absolute value of the value obtained by subtracting the lean air-fuel ratio from the rich air-fuel ratio may be acquired as the air-fuel ratio difference.

Then, the ECU 50 estimates the temperature of the three-way catalyst 31 (Step S7). Specifically, the temperature of the three-way catalyst 31 is estimated on the basis of a catalyst temperature map based on the acquired engine speed, the acquired engine load, and the air-fuel ratio difference. The catalyst temperature map is acquired by experiment in advance and is stored in the ROM of the ECU 50. The process of Step S7 is an example of the process which is performed by the estimation unit that estimates the temperature of the three-way catalyst 31 on the basis of the acquired operation state of the internal combustion engine 20 and the magnitude of the difference between the lean air-fuel ratio and the rich air-fuel ratio which are set as the target air-fuel ratios during execution of the catalyst regeneration control.

FIGS. 3A and 3B illustrate an example of the catalyst temperature map. The horizontal axis represents the engine speed, the vertical axis represents the engine load, and a plurality of isothermal lines are illustrated. FIG. 3A illustrates the catalyst temperature map when the air-fuel ratio difference is relatively large, and FIG. 3B illustrates the catalyst temperature map when the air-fuel ratio difference is relatively small. Here, temperatures T1 to T6 increase from the temperature T1 to the temperature T6.

As illustrated in FIGS. 3A and 3B, the temperature of the three-way catalyst 31 is estimated to be higher as the air-fuel ratio difference becomes larger under the condition in which the engine speed and the engine load are constant. The temperature of the three-way catalyst 31 during execution of the catalyst regeneration control can be accurately estimated on the basis of the catalyst temperature map. The temperature of the three-way catalyst 31 may be estimated on the basis of the engine speed, the engine load, and the air-fuel ratio difference using a calculation equation instead of the catalyst temperature map.

Then, the ECU 50 determines whether the estimated temperature of the three-way catalyst 31 is higher than a threshold value (Step S9) and this control ends when the determination result is negative. The threshold value is a value for determining whether the temperature of the three-way catalyst 31 increases excessively, which is set to a value slightly smaller than a heat-resistance upper-limit temperature of the three-way catalyst 31, and is, for example, 900 degrees, but is not limited thereto. The process of Step S9 is an example of the process which is performed by the determination unit that determines whether the estimated temperature of the three-way catalyst 31 is higher than the threshold value during execution of the catalyst regeneration control.

When the determination result in Step S9 is affirmative, the ECU 50 prohibits the catalyst regeneration control (Step S11). In a period in which the catalyst regeneration control is prohibited, the target air-fuel ratios of all the cylinders are set to be the same. Specifically, on the basis of the engine speed and the engine load acquired in Step S3, the target air-fuel ratios are set on the basis of the normal air-fuel ratio map which is used in the normal operation. Accordingly, an excessive increase in temperature of the three-way catalyst 31 is suppressed. In the period in which the catalyst regeneration control is prohibited, the target air-fuel ratios of all the cylinders may be set to the stoichiometric air-fuel ratio. The process of Step S9 is an example of the process which is performed by the prohibition unit that prohibits the catalyst regeneration control when it is determined that the estimated temperature of the three-way catalyst 31 is higher than the threshold value under execution of the catalyst regeneration control.

The regeneration prohibition control will be described below with reference to a timing chart. FIG. 4 is a timing chart illustrating an example of the regeneration prohibition control. In FIG. 4, waveforms indicating a vehicle speed, an idle determination flag, an engine speed, an engine load, an estimated temperature of the three-way catalyst 31, a catalyst regeneration execution flag, a target air-fuel ratio in a cylinder in which the target air-fuel ratio is set to the lean air-fuel ratio, and a target air-fuel ratio in a cylinder in which the target air-fuel ratio is set to the rich air-fuel ratio are illustrated. In FIG. 4, for the purpose of easy understanding, the estimated temperature of the three-way catalyst 31 is also illustrated in the period in which the catalyst regeneration control is not performed.

The catalyst regeneration execution flag is in an OFF state at time t1 and the catalyst regeneration execution flag is turned on when a catalyst regeneration request is issued at time t2. Accordingly, the ECU 50 performs the catalyst regeneration control of setting the target air-fuel ratio in one cylinder to the rich air-fuel ratio and setting the target air-fuel ratios in the other cylinders to the lean air-fuel ratio.

When the idle determination flag is turned on at time t3, the engine 10 is determined to be in an idle operation state and the catalyst regeneration execution flag is turned off to stop the catalyst regeneration control even when the catalyst regeneration request is issued. When the idle determination flag is turned off at time t4, the engine 10 is determined to depart from the idle operation state and the catalyst regeneration execution flag is turned on to perform the catalyst regeneration control again. When the idle determination flag is turned on again at time t5, the catalyst regeneration execution flag is turned off to stop the catalyst regeneration control.

When the idle determination flag is turned off at time t6, the catalyst regeneration execution flag is turned on to perform the catalyst regeneration control again. When the air-fuel ratio difference between the lean air-fuel ratio and the rich air-fuel ratio is increased after time t6 and the estimated temperature of the three-way catalyst 31 becomes higher than the threshold value at time t7, the catalyst regeneration execution flag is turned off during a predetermined period to prohibit the catalyst regeneration control. Thereafter, the idle determination flag is turned on at time t8 and the estimated temperature of the three-way catalyst 31 further decreases.

Since the catalyst regeneration control is prohibited during execution of the catalyst regeneration control as described above, the temperature of the three-way catalyst 31 decreases and an excessive increase in temperature is suppressed. Accordingly, it is possible to reduce a possibility of the three-way catalyst 31 degrading due to heat. When the catalyst regeneration control is prohibited, the state in which the catalyst regeneration control is prohibited is maintained in this trip until an ignition key is turned off. Accordingly, when a predetermined condition is satisfied during the next trip, the catalyst regeneration control can be performed to remove a sulfur compound deposited on the three-way catalyst 31.

Instead of estimating the temperature of the three-way catalyst 31 as in the above-mentioned technique, directly measuring the temperature of the three-way catalyst 31 using a temperature sensor or estimating the temperature of the three-way catalyst 31 using temperature sensors disposed upstream and downstream from the three-way catalyst 31 on the basis of the difference between the detected temperatures can be considered. However, in this case, there is a possibility of the number of components increasing. In this embodiment, since the temperature of the three-way catalyst 31 can be estimated without using such a temperature sensor, it is possible to suppress an increase in the number of components.

A modified example of the regeneration prohibition control will be described below. FIG. 5 is a flowchart illustrating a modified example of the regeneration prohibition control which is performed by the ECU 50. FIG. 6 is a timing chart illustrating a modified example of the regeneration prohibition control. The processes of Steps S1, S3, S5, S7, and S9 are the same as in the regeneration prohibition control illustrated in FIG. 2 and thus description thereof will not be repeated. The operation state such as the engine speed in a period from time t1 to time t7 is the same and thus description thereof will not be repeated.

As illustrated in FIG. 5, when the determination result in Step S9 is affirmative, the ECU 50 prohibits the catalyst regeneration control in a predetermined period (Step S11 a). The predetermined period is a period which is suitable for suppressing an excessive increase in temperature of the three-way catalyst 31 and is acquired by experiment in advance and stored in the ROM of the ECU 50. The predetermined period ranges, for example, from 500 ms to 1500 ms but is not limited thereto. When the period in which the catalyst regeneration control is prohibited elapses, the catalyst regeneration control is performed again in response to a catalyst regeneration request.

As illustrated in FIG. 6, at time t7′ at which a predetermined period elapsed from time t7 at which the catalyst regeneration control is prohibited, the prohibition of the catalyst regeneration control is released, the catalyst regeneration execution flag is turned on, and the catalyst regeneration control is performed again. When the idle determination flag is turned on at time t8 thereafter, the catalyst regeneration execution flag is turned off and the catalyst regeneration control is stopped.

The excessive increase in temperature of the three-way catalyst 31 can be suppressed by prohibiting the catalyst regeneration control in the predetermined period in this way, the temperature of the three-way catalyst 31 increases again when the prohibition of the catalyst regeneration control is released after the predetermined period elapses, and it is thus possible to remove a sulfur compound deposited on the three-way catalyst 31 as early as possible.

The predetermined period in which the catalyst regeneration control is prohibited is not limited to the above-mentioned example. For example, the predetermined period in which the catalyst regeneration control is prohibited may be a period until an idle operation flag is turned on after the catalyst regeneration control is prohibited. In a state in which the idle operation flag is turned on, the catalyst regeneration execution flag is turned off as described above. Accordingly, when the operation state becomes the idle operation state after the catalyst regeneration control is prohibited, the catalyst regeneration control can actually be performed only after the operation state is returned from the idle operation. The predetermined period in which the catalyst regeneration control is prohibited may be a period until the accelerator opening level becomes zero after the catalyst regeneration control is prohibited. In this case, since the catalyst regeneration control is not performed in a state in which the accelerator opening level is zero, the catalyst regeneration control can actually be performed only after the accelerator opening level becomes a value other than zero.

While an exemplary embodiment of the disclosure has been described above in detail, the disclosure is not limited to this specific exemplary embodiment, but can be modified in various forms without departing from the gist of the disclosure described in the appended claims.

In the above-mentioned embodiment, an in-line four cylinder engine has been described as an example of the internal combustion engine, but a V-type multi-cylinder engine including a catalyst for each bank may be employed. In this case, in the catalyst regeneration control, a target air-fuel ratio of one cylinder in each bank is set to a rich air-fuel ratio, target air-fuel ratios of the other cylinders are set to a lean air-fuel ratio, and the catalyst regeneration control is performed for each bank. 

What is claimed is:
 1. An exhaust gas control device for an internal combustion engine, the exhaust gas control device comprising: a catalyst configured to purify exhaust gas discharged from a plurality of cylinders of the internal combustion engine; and an electronic control unit configured to i) acquire an operation state of the internal combustion engine, ii) perform catalyst regeneration control, the catalyst regeneration control being control of increasing a temperature of the catalyst to regenerate the catalyst by setting a target air-fuel ratio of at least one cylinder among the plurality of cylinders to a rich air-fuel ratio which is lower than a stoichiometric air-fuel ratio and setting target air-fuel ratios of the other cylinders among the plurality of cylinders to a lean air-fuel ratio which is higher than the stoichiometric air-fuel ratio, iii) estimate the temperature of the catalyst based on the acquired operation state of the internal combustion engine and a difference between the lean air-fuel ratio and the rich air-fuel ratio which are set as the target air-fuel ratios during execution of the catalyst regeneration control, iv) determine whether or not the estimated temperature of the catalyst is higher than a threshold value during execution of the catalyst regeneration control, and v) prohibit the catalyst regeneration control when the electronic control unit determines that the estimated temperature of the catalyst is higher than the threshold value during execution of the catalyst regeneration control.
 2. The exhaust gas control device according to claim 1, wherein the electronic control unit is configured to prohibit the catalyst regeneration control in a predetermined period when the electronic control unit determines that the estimated temperature of the catalyst is higher than the threshold value during execution of the catalyst regeneration control.
 3. A control method for an exhaust gas control device for an internal combustion engine, the exhaust gas control device including a catalyst configured to purify exhaust gas discharged from a plurality of cylinders of the internal combustion engine and an electronic control unit, the control method comprising: i) acquiring an operation state of the internal combustion engine by the electronic control unit; ii) performing catalyst regeneration control by the electronic control unit, the catalyst regeneration control being control of increasing a temperature of the catalyst to regenerate the catalyst by setting a target air-fuel ratio of at least one cylinder among the plurality of cylinders to a rich air-fuel ratio which is lower than a stoichiometric air-fuel ratio and setting target air-fuel ratios of the other cylinders among the plurality of cylinders to a lean air-fuel ratio which is higher than the stoichiometric air-fuel ratio; iii) estimating, by the electronic control unit, the temperature of the catalyst based on the acquired operation state of the internal combustion engine and a difference between the lean air-fuel ratio and the rich air-fuel ratio which are set as the target air-fuel ratios during execution of the catalyst regeneration control; iv) determining, by the electronic control unit, whether or not the estimated temperature of the catalyst is higher than a threshold value during execution of the catalyst regeneration control; and v) prohibiting, by the electronic control unit, the catalyst regeneration control when the electronic control unit determines that the estimated temperature of the catalyst is higher than the threshold value during execution of the catalyst regeneration control. 